Experiments are proposed to investigate a novel molecular approach for labelling ensembles of physiologically active neurons in the olfactory system. It is based on the finding that diverse sensory and neuronal stimulation can increase expression of the proto-oncogenes, c-fos and c- jun. Fos and Jun, the respective protein products of these genes, are present in most cells and can be detected using immunohistochemistry. The project builds on preliminary evidence that Fos-like immunoreactivity occurs in olfactory tissue and can be developed into a suitable probe for the planned studies. Experiments will determine if odorants can produce c- fos and c-jun induction in receptor cells and in higher-order olfactory neurons. Subsequent investigations will determine the odor intensity and specificity required to produce this labelling and will examine the time course of this induction. Compared to previously used ensemble labelling techniques, this method has the capacity to provide superior cellular resolution in labelling physiologically active neurons and would provide a powerful new way of investigating pattern discrimination in olfaction.